1. Field of the Invention
This invention relates to the art of papermaking, particularly to treating paperboard produced from SCMP and sulfite pulps with pressure and heat to improve its wet strength while preserving its folding endurance.
2. Description of the Prior Art
The semichemical mechanical pulping process is a method of production of an aqueous slurry of fibers by treatment of a suitable renewable raw material. In most pulping processes, a considerable portion of the natural natural lignin in wood, grass or other vegetative matter is rendered soluble by chemical reaction with one or more nucleophilic reagents. Minimization of the lignin portion solubilized and removed whilst so altering the lignin as to permit recovery of fibers by the mechanical action of a disk or other refiner or shredder in a condition of little damage is the goal of the SCMP process. It is also the goal of certain related processes known as the chemimechanical process (CMP), the chemithermomechanical process (CT-MP) and the neutral sulfite semichemical (NSSC) processes. Such pulps are normally considered to be more brittle and of inferior strength when compared to lower lignin pulps produced by the kraft, sulfite, kraft-anthraquinone (AQ), soda-AQ or alkaline sulfite AQ processes. However, properties are adequate for many end-uses, including corrugated medium and even as a linerboard component. Such pulps gain wet strength, without the severe enhancement of brittleness that is caused by heat treatment alone, if densified before or during the heat treatment.
In the sulfite process, sulfite or bisulfite ion is the nucleophilic agent. The sulfite or bisulfite ions cause the lignin molecules to break into small fragments. During this chemical reaction, the sulfite or bisulfite ions become chemically bonded to the lignin fragments thereby providing water solubility. A variation of the sulfite process involves the use of anthraquinone (AQ) or substituted anthraquinones as a second nucleophile. AQ is reduced in situ during the earliest stages of the cook to anthrahydroquinone (AHQ). As AQ is insoluble and only the salt from of AHQ is soluble, alkali presence is necessary for solution formation and uniform penetration of AQ into the wood chip, grass stem or any other fiber-containing vegetative matter. Such a cooking process is known as an alkaline sulfite-AQ process. Both the sulfite process and the alkaline sulfite-AQ variation of the sulfite process are well known to the industry and pulps thus prepared can be used to give the benefits of our invention.
In the art of making SCMP linerboard, it is conventional to subject felted fibers to wet pressing to unite the fibers into a coherent sheet. Pressure is typically applied to a continuous running web of paper by a series of nip rolls which, by compressing the sheet, both increase its volumetric density and reduce its water content. The accompanying FIG. 1 shows in simplified diagrammatic form a typical papermaking machine, including a web former and three representative pairs of wet press rolls. Also shown are drying rolls whose purpose is to dry the paper to a desired final moisture content, and a calendar stack to produce a smooth finish. At least some of the rolls are ordinarily heated to hasten drying. (The drawing is simplified--there many more drying rolls in actual practice.)
There is currently considerable interest in treatments involving heat and pressure, or heat alone, during or after the production process, to improve various qualities of paperboard. Quantifiable board qualities include dry tensile strength, wet tensile strength, reverse folding endurance, compressive strength and stiffness, among others. Which qualities should desirably be enhanced depends upon the intended application of the product. For linerboard to be used in manufacturing corrugated cartons for use in humid or wet environments, three qualities of particular interest are wet strength, folding endurance and high humidity compression strength, all of which can be measured by well-known standared tests. As used herein, then, "wet strength" means wet tensile strength as measured by American Society for Testing and Materials (ASTM) Standard D829-48. "Folding endurance" is defined as the number of times a board can be folded in two directions without breaking, under conditions specified in Standard D2176-69. "Basis weight" is the weight per unit area of the dried end product.
Prior workers in this field have recognized that high-temperature treatment of linerboard can improve its wet strength. See, for example E. Back, "Wet stiffness by heat treatment of the running web", Pulp & Paper Canada, vol. 77, No. 12, pp. 97-106 (December 1976). This increase has been attributed to the development and cross-linking of naturally occurring polysaccharides and other polymers, which phenomenon may be sufficient to preserve product wet strength even where conventional synthetic formaldehyde resins or other binders are entirely omitted.
It is important to note that wet strength improvement by heat curing has previously been thought attainable only at the price of increased brittleness (i.e., reduced folding endurance). Therefore, most prior high-temperature treatments have been performed on particle board, wallboard, and other products not to be subjected to flexure. The known processes, if applied to paperboard, would produce a brittle product. Embrittled paperboard is not acceptable for many applications involving subsequent deformation such as the converting operation on a corrugating machine to make corrugated boxes out of linerboard, and therefore heat treatment alone, to develop wet strength of linerboard, has not gained widespread acceptance. As Dr. Back has pointed out in the article cited above, "The heat treatment conditions must be selected to balance the desirable increase in wet stiffness against the simultaneous embrittlement in dry climates." Significantly, in U.S. Pat. No. 3,875,680, Dr. Back has disclosed a process for heat treating already manufactured corrugated board to set previously placed resins, the specific purpose being to avoid running embrittled material through a corrugator.
It is plain that added wet strength and improved folding endurance were previously thought incompatible results.
It is therefore an object of the invention to produce paperboard having both greatly improved wet strength and good folding endurance. Another goal is to achieve that objective without resorting to synthetic resins or other added binders and wet strength agents.
With a view to the foregoing, a process has been developed which dramatically and unexpectedly increases not only the wet strength of paperboard, but also preserves its folding endurance. In its broadest sense, the invention comprises steps of (1) subjecting paperboard produced from SCMP pulp to high pressure densification, and (2) heating the board to an internal temperature of at least 420.degree. F. (216.degree. C.) for a period of time sufficient to increase the wet strength of the board.
This method produces a product having folding endurance greatly exceeding that of similar board whose wet strength has been increased by heat alone. This is clearly shown by our tests exemplified below.
While the tests set out in Examples 1-2 have carried out the invention in a static press, it is preferred that the heat and pressure be applied to continuously running board by hot pressure rolls inasmuch as much higher production rates can be attained.
We prefer to raise the internal temperature of the board to at least 450.degree. F. (232.degree. C.), as greater wet strength is then achieved. This may be because at higher temperatures, shorter step duration is necessary to develop bonding, and there is consequently less time for fiber degradation to occur. Also, shorter durations enable one to achieve higher production speeds.
It should be noted that the heating rate, and thus the required heating duration at a particular temperature, depends on method of heat transfer chosen. Furthermore, it is desirable to raise the web temperature as rapidly as possible to the chosen treating temperature. Improved heating rates can be achieved by using high roll temperatures and/or by applying high nip forces to the press roll against the sheet on the hot rolls. That high pressure dramatically improves heat transfer rates has previously been disclosed. One worker has attributed this to the prevention of vapor formation at the web-roll interface.
While the invention may be practiced over a range of temperatures, pressures and durations, these factors are interrelated. For example, the use of higher temperatures requires a heating step of shorter duration, and vice-versa. At 450.degree. F., a duration of 5 seconds has been found sufficient to obtain substantial improvements, while at lower temperature, considerably longer time is required to achieve the same improvement.
It is presently preferred that, for safety reasons, the roll temperature be not greater than the web ignition temperature (572.degree. F., 300.degree. C.); however, even higher roll temperatures may be used if suitable precautions, such as the provision of an inert atmosphere, or rapid removal of paper from the hot environment, are taken.